The present invention relates generally to vehicle powertrain systems and, in particular, to a hydraulic hybrid powertrain system.
So-called hybrid powertrains, such as for automotive vehicles, generally refer to a powertrain wherein an internal combustion engine is utilized in combination with an auxiliary motor, such as electric motor or a hydraulic motor, to drive the vehicle. Hybrid powertrain systems known as parallel hybrids include a typical mechanical drivetrain (coupled to the internal combustion engine) along with the auxiliary drivetrain (coupled to the auxiliary motor). These systems are disadvantageously high in weight because of the necessary duplication of parts. Hybrid drive systems known as series hybrids do away with the mechanical powertrain and drive the vehicle solely by a hydraulic motor or motors while utilizing an engine to provide the necessary hydraulic pressure for the hydraulic motor. These systems are more attractive because of the potential reduction in weight and resultant efficiency gains. While the attractiveness of such a hydraulic hybrid powertrain has been recognized, there remain many efficiency issues regarding the operation and control of the engine with respect to the hydraulic drive motor.
It is desirable, therefore, to provide a hydraulic hybrid powertrain system that provides increased efficiency of the entire hydraulic hybrid powertrain system.